Nanoscale array structures suitable for surface enhanced raman scattering and methods related thereto

ABSTRACT

Methods for fabricating nanoscale array structures suitable for surface enhanced Raman scattering, structures thus obtained, and methods to characterize the nanoscale array structures suitable for surface enhanced Raman scattering. Nanoscale array structures may comprise nanotrees, nanorecesses and tapered nanopillars.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationNo. 61/266,036, filed on Dec. 2, 2009, which is incorporated herein byreference in its entirety.

STATEMENT OF GOVERNMENT GRANT

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

FIELD

The present disclosure relates to microstructures and their fabricationmethods. In particular, it relates to array structures suitable forsurface enhanced Raman scattering and methods related thereto.

BACKGROUND

Raman scattered light is considered a chemical signature of chemical andbiological molecules since all molecules with unique chemicalcompositions have unique Raman scattering spectra. Raman scatteringspectroscopy is thus a powerful technique to detect chemical andbiological molecules without labeling and has potential applications inhigh-sensitivity detections of explosives, pathogens, and contaminantsin the field. Unfortunately, Raman scattering is an extremelyinefficient process due to its weak sensitivity as compared to otherscattering mechanisms (roughly only 1 in 10⁸ photons ends up in Ramanscattering) [Jarvis and Goodacre, 2004 Anal. Chem. 76 40] and also dueto typical lower scattering cross-sections of Raman process (˜10⁻³⁰cm²), which is around 15 orders of magnitude lower than fluorescenceemission [Vo-Dinh et al., 2002 J. Raman Spectrosc. 33 511]. In order toget detectable Raman scattering, it is necessary to use an array offiltering techniques or to enhance the Raman scattering process. Thelatter of the two can be achieved using surface enhanced Ramanscattering, which is also known as surface enhanced Raman scatteringspectroscopy or surface enhanced Raman spectroscopy. For the sake ofclarity, throughout the present disclosure, the term “SERS” intends toindicate surface enhanced Raman scattering.

Back in the 1970's, many scientists came to discover a new phenomenon inRaman scattering, which is now widely referred to as SERS. When chemicaland bio-molecules are adsorbed on a roughened noble metal surface, theRaman scattering light can be enormously amplified and the sensitivityof the Raman spectroscopy enhanced by several orders of magnitude. Oneof the many approaches that has been tried includes microscale ornanoscale “roughening,” such as in the form of electrochemical texturingof a surface before metal sputtering [Murray et al., 1981 Phys. Rev.Lett. 46 57]. In general, reported signal enhancements have beensignificant. However, enhancement is observed only at so-called “hotspots,” regions where the Raman signal is higher, while neighboringregions might exhibit little or even no significant signal enhancement.

For the sake of clarity, throughout the present disclosure, the term“hot spot”, “hot spots”, or “SERS hot spots” intends to indicate regionswhere the Raman signal is higher, or enhanced. The low concentration ofhot spots within a sample is exacerbated further by an inconsistency ofperformance between different but essentially identical substrates, orsample-to-sample uniformity [Netti et al., 2005 Raman Technology forToday's Spectroscopists]. Overall, these two aspects togethereffectively have prevented SERS from being widely recognized as aquantifiable spectroscopic technique [Etchegoin and Le Ru, 2008 Phys.Chem. Chem. Phys. 10 6079-6089].

SUMMARY

According to a first aspect, a structure is provided, comprising: aplanar substrate; an array of nanopillars on the planar substrate, thearray of nanopillars substantially perpendicular to the planar substrateand having a substantially cylindrical shape; a metal drop having asubstantially spherical shape on each nanopillar of the array ofnanopillars; and a second metal covering a surface of the planarsubstrate.

According to a second aspect, a structure is provided, comprising: aplanar substrate; an array of nanorecesses in the planar substrate; anda first metal covering the planar substrate and the array ofnanorecesses.

According to a third aspect, a structure is provided, comprising: aplanar substrate; an array of tapered nanopillars on the planarsubstrate, wherein each tapered nanopillar of the array of taperednanopillars is substantially perpendicular to the planar substrate andhave a substantially conical shape, the conical shape being wider on aside connected with the planar substrate and tapered on a side oppositethe planar substrate; and a metal layer covering a surface of the planarsubstrate and covering each tapered nanopillar of the array of taperednanopillars.

According to a fourth aspect, a structure is provided, comprising: aplanar substrate; an array of straight nanopillars on the planarsubstrate, wherein each straight nanopillar of the array of straightnanopillars is substantially perpendicular to the planar substrate andhave a substantially cylindrical shape; and a metal layer covering asurface of the planar substrate and covering each straight nanopillar ofthe array of straight nanopillars.

According to a fifth aspect, a method for fabricating a structure isprovided, the method comprising: providing a planar substrate; coatingthe planar substrate with a polymer layer; annealing the polymer layer,wherein the annealing causes a microphase separation of the polymerlayer into first portions and second portions, the first portions beingan array of nanostructures surrounded by the second portions, the secondportions being a polymer matrix; removing the array of nanostructures,whereby the removing forms an array of nanorecesses and the removingexposes portions of the planar substrate below the removednanostructures; and coating a first metal on the polymer matrix and theexposed portions of the planar substrate.

According to a sixth aspect, a method for fabricating a structure isprovided, the method comprising: providing a planar substrate; coating aphotoresist layer on the planar substrate; exposing the photoresistlayer with laser light, the laser light forming a spatial interferencepattern on the photoresist layer; developing the photoresist layer toform an array of photoresist islands according to the spatialinterference pattern; removing portions of the planar substrate notcovered by the array of photoresist islands, wherein the removing formsan array of tapered nanopillars on the planar substrate; removing thearray of photoresist islands; and coating a metal layer on the array oftapered nanopillars.

According to a seventh aspect, a method for measuring a lightenhancement factor for an array structure in surface enhanced Ramanscattering is provided, the method comprising: providing the structureaccording to the first aspect; contacting a plurality of molecules withthe structure, the plurality of molecules being on the metal drop and onthe second metal; and measuring a near field scattering intensity fromthe structure with a near field scanning optical microscope, thusmeasuring the light enhancement factor for the structure in surfaceenhanced Raman scattering (SERS).

According to an eighth aspect, a method for measuring a local electricfield intensity with nanometer resolution for an array structure insurface enhanced Raman scattering, the method comprising: providing thestructure according to the first aspect; placing a plurality ofsemiconductor quantum dots or artificial fluorophore molecules in aplurality of hot spots in the structure; and measuring aphotoluminescence and/or a fluorescence spectroscopy and calculating theexpected local electric field intensity based on quantum-confined Starkeffect, thus measuring the local electric field intensity with nanometerresolution for the structure.

Further embodiments of the present disclosure can be found in thewritten specification, drawings and claims of the present application.According to some embodiments of the present disclosure, the teachingsof the present disclosure provide methods for fabricating nanoscalearray structures suitable for surface enhanced Raman scattering,structures thus obtained, and methods to characterize the nanoscalearray structures suitable for surface enhanced Raman scattering.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of this specification, illustrate one or more embodiments of thepresent disclosure and, together with the description of exampleembodiments, serve to explain the principles and implementations of thedisclosure.

FIGS. 1A-1J show fabrication steps of a structure in accordance with anembodiment of the present disclosure. In particular:

FIG. 1A shows an exemplary polymer layer on a planar substrate.

FIG. 1B shows an exemplary array of nanostructures in a polymer matrixon the planar substrate.

FIG. 1C shows a perspective view of the exemplary array ofnanostructures in a polymer matrix on the planar substrate.

FIG. 1D shows an exemplary array of nanorecesses in a polymer matrix onthe planar substrate.

FIG. 1E shows an exemplary array of nanorecesses in a polymer matrix onthe planar substrate covered by a first metal.

FIG. 1F shows an exemplary array of metal islands on the planarsubstrate.

FIG. 1G shows an exemplary array of metal drops on the planar substrate.

FIG. 1H shows an exemplary array of metal drops on an array ofnanopillars on the planar substrate.

FIG. 1I shows an exemplary array of metal drops on an array ofnanopillars on the planar substrate covered by a second metal.

FIG. 1J shows an exemplary array of metal drops on an array ofnanopillars on the planar substrate after a second anneal.

FIGS. 2A-21 show fabrication steps of a structure in accordance withanother embodiment of the present disclosure. In particular:

FIG. 2A shows an exemplary photoresist layer on an exemplaryantireflective layer on a planar substrate.

FIG. 2B shows an exemplary photoresist layer on an exemplaryantireflective layer on a planar substrate after exposure with laserinterference lithography.

FIG. 2C shows a perspective view of the exemplary photoresist layer onan exemplary antireflective layer on a planar substrate after exposurewith laser interference lithography.

FIG. 2D shows an exemplary array of photoresist islands on a planarsubstrate after exposure with laser interference lithography anddevelopment.

FIG. 2E shows an exemplary array of photoresist islands on a planarsubstrate after exposure with laser interference lithography,development, and some removal of the exposed portions of the planarsubstrate.

FIG. 2F shows an exemplary array of photoresist islands on a planarsubstrate after exposure with laser interference lithography,development, and additional removal of the exposed portions of theplanar substrate.

FIG. 2G shows an exemplary array of photoresist islands on a planarsubstrate after exposure with laser interference lithography,development, and after completion of the removal of the exposed portionsof the planar substrate.

FIG. 2H shows an exemplary array of tapered nanopillars on a planarsubstrate after the remaining photoresist islands are removed.

FIG. 2I shows an exemplary array of tapered nanopillars on a planarsubstrate coated with a metal layer.

FIG. 3 shows an exemplary laser interference lithography system.

FIG. 4 shows an exemplary plasmon nano-capacitor structure.

FIG. 5 shows an exemplary detailed view of an array structure suitablefor SERS.

FIG. 6 shows an exemplary far field scanning confocal Ramanmicrospectroscopy system capable of SERS measurements.

FIG. 7 shows an exemplary SERS measurement showing enhancement factorand uniformity.

DETAILED DESCRIPTION

The applicants disclose methods to create array structures suitable forSERS which addresses the two known limitations of SERS substratesaccording to current art: the low concentration of “hot spots” wheresignificant signal enhancement is seen, and the consistency anduniformity of performance from sample to sample. The applicants disclosethe application of highly uniform batch nano-fabrication methods: blockcopolymer nano-lithography and laser interference lithography tofabricate uniform nano scale patterns on wafer size samples, as well asthe subsequent processing to create highly uniform nanoscale plasmonicantenna array structures suitable for SERS with high concentration ofhot spots. The applicants also disclose methods to characterize thearray structures suitable for SERS, confirming significant improvementin the local electric field intensity, the concentration of hot spotsand the consistency of performance over current art by measuring theenhancement factor and its repeatability.

First, the applicants disclose the design of a plurality of nanoscaleplasmonic antenna array structures suitable for SERS. Plasmons describethe collective free electron oscillations in metallic nanostructuresupon excitation by external electromagnetic field. That is, the freeelectrons in metal may be driven by optical excitations and, at theplasmon resonance frequency, the free electron oscillation can maintainthe highest kinetic energy level. If the applicants carefully design ananoscale plasmonic antenna array to “squeeze” most of the freeelectrons to an extremely small area and bring two such areas very closeto each other, the applicants can artificially make a plasmonnanocapacitor structure (410) as illustrated in FIG. 4 with a hot spot(420) in the center formed by the two triangles shown in FIG. 4. Inpractice, the hot spot (420) of the nanocapacitor structure (410) maynot have to be formed by two triangular shapes but may be formed by twocurved surfaces or two flat surfaces with pointed or curved featuresthat are close to each other.

By mimicking the structure of a tree and forest, applicants createarrays of metal topped nanopillars such as nanotrees and nanoforest toact like optical antennas that can collect far field optical excitation,generate both localized and surface resonant plasmons, and create SERShot spots for SERS. The term “ hot spots” have previously been definedin the Background Section of the present disclosure as locations whereRaman signal enhancement is expected. The illustration of individualnanotrees is depicted in FIG. 5. The nanotree (510) can be asubstantially spherical or hemispherical metal drop on the nanotrunk,which can be a short silicon nanopillar (520) as shown in FIG. 5. Thenanotrees can be arranged side by side with a gap width (530) of about50 nm or less from each other to form a high-density array of nanotreesor nanoforest. A gap width of about or less than 10 nm is preferable forSERS. The ground substrate underneath the nanoforest can be covered witha metal thin film (540). In the present disclosure, the applicants alsodisclose a structure comprising an array of metal covered taperedpillars, a structure comprising metal covered straight pillars, and yetanother structure comprising an array of metal covered nanodimples ornanorecesses.

For the sake of clarity, throughout the present disclosure, the term“SERS substrate” intends to indicate any substrate onto which surfaceenhanced Raman scattering occurs when molecules are contacted to thesubstrate. The nanoscale array structures disclosed in the presentdisclosure are all suitable for SERS and therefore each nanoscale arraystructure can be a SERS substrate. Also for the sake of clarity,throughout the present disclosure, the term “hot spot”, “hot spots”, or“SERS hot spots” intends to indicate regions where the Raman signal ishigher, or enhanced, as previously stated in the Background Section ofthe present disclosure.

The applicants note that these three-dimensional (3D) arrayednanodevices serve as excellent SERS substrates due to the followingadvantages. First, the gaps between adjacent substantially sphericalmetal nanotrees (510) are potential SERS hot spots (550), which resemblethe typical SERS hot spots created in aggregated nanoparticle clustersin current art while maintaining a higher uniformity and order than incurrent art. Second, the rims of the metal covered arrays ofnanorecesses can also serve as the SERS hot spots which resemble thecurrent art of nano-ring devices [Aizpurua et al., 2003 Phys. Rev. Lett.90 057401-057404]. Third, the gaps between the nanotrees (510) and themetal thin film (540) on the ground substrate can be SERS hot spots(550) also, due to the inter-coupling of the localized plasmon in thenanotree (510) and the surface plasmon on the metal thin film (540)covering the substrate. Lastly, the plasmon resonance of the proposedarray of nanotrees may be much stronger than that of individualnanotrees or randomly ordered array of nanostructure due to coupling.

The nanoscale array structures suitable for SERS of the presentdisclosure also have the additional advantage of being tunable. Thestructures can be tuned by varying, for example, gap width, height ofpillars, or use of dielectric fill materials of various refractiveindexes in the gaps, the details of which is discussed in Bora et al.,2010 Nano Lett. 10 2832-2837 the content of which is incorporated hereinby reference in its entirety [Bora et al., 2010 Nano Lett. 102832-2837]. The pitch (e.g., distance from the center of the nanotree(510) to the center of the next nanotree (510)) of the structures canalso be changed by, for example, the use of lasers with differentcharacteristic wavelengths during laser interference lithography.

Furthermore, the 3D nanoscale array structures of the present disclosurecan also be used in other applications besides SERS. Possibleapplications, by way of example and not of limitation, include solarcells, energy harvesting devices, plasmonic nanolasers, tunable sources,supercapacitors and batteries, as well as plasmonic based chemical andbiosensors.

The applicants note that the nanotree of the present disclosure can be aplasmon photonic crystal whose plasmon resonance frequency and strengthare dependent on not only the individual nanoparticle but also theinterparticle coupling. The metal thin film on the substrate can be alsoa classic plasmon photonic crystal structure with the plasmon resonancefrequency and strength dependent on the gap width. This 3D plasmonphotonic crystal structure can permit sharper and stronger plasmonresonance with less inhomogeneous broadening and energy damping.Furthermore, the 3D array of nanotrees can contain higher density of hotspots per unit area than any previously demonstrated SERS substratedevices. In practical applications, molecules can have higherpossibility to be at the position of the SERS hot spots due to thehigher density of hot spots.

In the present disclosure, the applicants describe several large-areahighly uniform tunable nanoscale array structures suitable for SERS andmethods of fabricating the array structures. Also described in thepresent disclosure is the result of optical characterization, whichverifies an exemplary embodiment of a tapered nanopillar array structuresuitable for SERS by quantifying the Raman enhancement factor,uniformity and repeatability. An average homogeneous enhancement factorof well into the 10⁷ range was obtained for an exemplary embodiment ofbenzenethiol molecules adsorbed on the tapered nanopillar arraystructure substrate [Garda et al., 2010 Nanotechnology 21395701-395709]. In what follows, array structures suitable for surfaceenhanced Raman scattering, fabrication methods thereof, andcharacterization methods thereof are described in accordance withvarious embodiments of the present disclosure.

FIGS. 1A-1J show one embodiment of the steps of fabricating an arraystructure suitable for SERS in accordance with the present disclosure.

FIG. 1A shows a cross-sectional view of a planar substrate (110) coatedwith a polymer layer (120). By way of example and not of limitation, theplanar substrate (110) as described in the present disclosure is made ofsilicon dioxide substrate, but the planar substrate (110) can be made ofother materials such as quartz, fused silica, or sapphire. The planarsubstrate (110) can also be made of single element or compoundsemiconductors such as silicon, germanium, or gallium arsenide. Ingeneral, the planar substrate (110) can be of any material that does notfluoresce or otherwise generate large background signal in the spectralregion of interest

The polymer layer (120) comprises two or more chemically differentpolymer blocks that are chemically bonded together on each molecularchain but can microphase separate and form one or more periodic array ofnanostructures (130) of FIG. 1B due to chemical incompatibility betweenthe two or more blocks. The microphase separation is analogous to theseparation of oil and water. The periodic array of nanostructures (130)formed from the polymer layer (120) can be substantially spherical,cylindrical, or lamellar. The polymer layer (120) can be a blockcopolymer. The polymer layer can specifically be made ofpolystyrene-poly(methyl methacrylate),polystyrene-polybutadiene-polystyrene, or other polymers.

FIG. 1B shows a cross-sectional view of the polymer layer (120) of FIG.1A on the planar substrate (110) after annealing. The annealing processheats the polymer layer (120) of FIG. 1A to a temperature high enough toallow the molecular chains to move and the blocks to microphase separateand form periodic nanostructures (130), which are thermodynamicallyfavorable.

For example, FIG. 1C shows a first portion of the polymer layer (120) ofFIG. 1A forming an array of nanostructures (130) with substantiallyspherical shape while a second portion of the polymer layer (120) ofFIG. 1A forms a polymer matrix (140) surrounding the array ofnanostructures (130). FIG. 1C shows a perspective view of the annealedpolymer layer (120) of FIG. 1A after annealing whereby the microphaseseparation forms the array of nanostructures (130) and the polymermatrix (140).

FIG. 1D shows a cross-sectional view of the annealed polymer layer (120)on the planar substrate (110) where the array of substantially sphericalnanostructures (130) has been removed. Removal can be done by reactiveion etch or other methods. After the array of nanostructures (130) hasbeen removed, the polymer matrix (140) remains and surrounds an array ofnanoscale recesses or nanorecesses (135).

The resulting array of nanorecesses (135) is highly uniform, and cancover areas as large as a whole wafer many inches in diameter (e.g., 4,6 or 12 inches). Each of the nanorecesses (135) of the array ofnanorecesses (135) can have an associated exposed portion of the planarsubstrate (150) below each of the removed nanostructures (130).

FIG. 1E shows a cross-sectional view of the array of nanorecesses (135)on the planar substrate (110) coated by a first metal (160). The firstmetal (160) can be silver, gold, aluminum, iridium platinum, palladium,copper or any other metal or metal alloy. The first metal (160) coatsthe top surface of the polymer matrix (140), and also the exposedportions of the planar substrate (150) below each of the removednanostructures. The coating by the first metal (160) can be done byelectrochemical deposition, electron beam evaporation (angled ororthogonal), physical sputtering, atomic layer deposition or othermethods. The structure shown in FIG. 1E is an array of nanorecesses(135) suitable for SERS that can be referred to as nanodimples.

The array of nanorecesses (135) from FIG. 1E can be utilized as a SERSsubstrate fabricated with air covering and filling the nanorecesses(135). Other dielectric materials, such as silicon dioxide can be usedto cover and fill the nanorecesses (135) to tune the array structuresuitable for SERS.

FIG. 1F shows a cross-sectional view of the array of FIG. 1E after thepolymer matrix (140) and the portion of the first metal (160) in FIG. 1Eon the polymer matrix (140) have been removed. The remaining portion ofthe first metal (160) in FIG. 1E forms an array of metal islands (170)on the planar substrate (110). The removal of the polymer matrix (140)and the portion of the first metal (160) in FIG. 1E on top of thepolymer matrix (140) can be accomplished by methods such as reactive ionetch, ion milling, or aqueous or solvent based wet removal methods.

FIG. 1G shows a cross-sectional view of the array of metal islands (170)of FIG. 1F on the planar substrate (110) after annealing to reflowtemperature, whereby the array of metal islands (170) of FIG. 1F nowforms an array of metal drops (180) on the planar substrate (110). Themetal drops (180) form a mask for a subsequent removal or etchingprocess whereby portions of the planar substrate (110) not covered bythe metal drops (180) are etched or removed. As a result of the etchingor removing, an array of nanopillars (185) is formed under the array ofmetal drops (180) from the remaining planar substrate (110) as shown inFIG. 1H. In particular, FIG. 1H shows a cross-sectional view of thearray of nanopillars (185) of FIG. 1H after the etching or removalprocess.

FIG. 1I shows a cross-sectional view of the array nanopillars (185) ofFIG. 1H after a coating of a second metal (190) which covers the metaldrops (180) on top of each of the nanopillars and the exposed portionsof the planar substrate. The second metal (190) can be silver, gold,aluminum, iridium platinum, palladium, copper or any other metal. Thesecond metal (190) can be of the same material as the first metal (160)in FIG. 1E. The coating by the second metal (190) can be done byelectrochemical deposition, electron beam deposition (angled ororthogonal), physical sputtering, atomic layer deposition, or othermethods.

FIG. 1J shows a structure formed by the array of metal drop (180)covered nanopillars and the second metal (190) covered planar substrate(110) from FIG. 1H after a second annealing process. The array of metaldrops (180) coated by the second metal (190) is heated again in thesecond annealing process to reflow whereby a shape change occurs in eachmetal drop (180) of the array of metal drops (180). The metal drops(180) have a gap width (195) of about 50 nm or less and preferably about10 nm or less. The metal drops (180) can be substantially spherical inshape.

The array of metal drop (180) covered nanopillars (185) from FIGS. 1H-1Ican be utilized as a SERS substrate as fabricated with air covering andfilling an area between the metal drops (180) and between thenanopillars (185) in FIG. 1H. Other dielectric materials, such assilicon dioxide, can be used to cover and fill in between the metaldrops (180) and in between the nanopillars (185) to tune the arraystructure suitable for SERS.

FIGS. 1A-1J show an embodiment of the steps of fabricating an arraystructure suitable for SERS in accordance with the disclosure. Theperson skilled in the art will understand that the number of such stepsis only indicative and that the process can occur in more or fewer stepsaccording to the various embodiments. For example, the metal drops (180)can be formed from a single metal layer rather than a first metal (160)and a second metal (190).

FIGS. 2A-2I show another embodiment of the steps of fabricating an arraystructure suitable for SERS in accordance with the disclosure.

FIG. 2A shows a planar substrate (210) coated with an antireflectivelayer (230), and the antireflective layer (230) is coated with aphotoresist layer (220). The antireflective layer (230) is an optionallayer which serves to improve the effective resolution of thephotoresist layer (220) interaction with the light source by reducing orremoving the reflection from the substrate (210). For somenon-reflective substrates or larger dimension process, theantireflective layer (230) may be optional. In an embodiment of thepresent disclosure, an average 0.45 μm thick photoresist layer (220) isused. More details regarding the planar substrate (210) are disclosed inthe description of FIG. 1A.

FIG. 2B shows the photoresist layer (220) in FIG. 2A on theantireflective layer (230) on the planar substrate (210) after thephotoresist layer (220) in FIG. 2A has been exposed with a spatialinterference pattern (320) formed by laser light (310), shown in FIG. 3in a process known as laser interference lithography [Fernandez et al.,1997 J. Vac. Sci. Technol. B 15 (3) 729-735].

Referring now to FIG. 3, laser interference lithography is describedgraphically. Laser interference lithography can be conducted bysplitting the laser light (310) from a single source into two beams.Each of the two beams is spread with optical components and thenrecombined to create a two dimensional spatial interference pattern(320) by the constructive and destructive interference of the beams. Theconstructive interference forms bright regions where the two beams areadded, and the destructive interference forms dark regions where the twobeams cancel one another. The spatial interference pattern (320) is thenused to expose the photoresist layer (220) in FIG. 2A. Although a singlesource of laser light (310) is shown here, two separate sources can beused. In an embodiment of the present disclosure, 413 nm wavelengthlaser interference illuminated with a dose of ˜40 mJ/cm² is used [Gartiaet al., 2010 Nanotechnology 21 395701-395709].

Returning to FIG. 2B, the photoresist layer (220) in FIG. 2A undergoinga chemical change upon exposure by the spatial interference pattern(320) is shown. The exposed portions of the photoresist layer (220) inFIG. 2A undergo a chemical change which causes the exposed portions tobecome either soluble or insoluble in the developer solution as comparedto the unexposed portion depending on the tone of the resist (e.g.,positive or negative). Therefore, the exposure converts the photoresistlayer (220) in FIG. 2A into two portions: the first portion is an arrayof photoresist islands (240) and the second portion is a photoresistmatrix (245) which surrounds the array of photoresist islands (240). Oneportion is typically more soluble in the developing solution than theother. The photoresist islands (240) are typically cylindrical in shape.

FIG. 2C shows a perspective view of the exposed photoresist layer (220)in FIG. 2A showing the array of photoresist islands (240) in aphotoresist matrix (245). In an embodiment of the present disclosure,the photoresist islands (240) are cylindrical in shape and areindividually 150 nm in diameter and 350 nm in pitch [Gartia et al., 2010Nanotechnology 21 395701-395709]. The pitch of the photoresist islands(240) (e.g., distance from the center of the photoresist island (240) tothe center of the next photoresist island (240)) can be changed or tunedby utilizing a different type of laser source. Also, the size of each ofthe photoresist islands (240) can be adjusted by selecting or developinga photoresist with a nonlinear response to light exposure.

FIG. 2D shows the array of photoresist islands (240) remaining after theexposed photoresist layer (220) in FIG. 2A is developed by applying adevelopment solution which dissolves the photoresist matrix (245)selectively.

FIG. 2E shows portions of the planar substrate (210) partially removedby a removal process such as ion milling, reactive ion etch, or wetetch. The portions of the planar substrate (210) covered by the array ofphotoresist islands (240) are protected while the exposed portions ofthe planar substrate (210) are removed. In an embodiment of the presentdisclosure, the removal of the exposed portions of the planar substrate(210) is done by an ion milling deep reactive ion etch by a highlydirectional Bosch process [Garda et al., 2010 Nanotechnology 21395701-395709].

FIGS. 2E-2G show the progression of the removal process for the planarsubstrate (210) from FIG. 2E to FIG. 2F to FIG. 2G where each island ofthe array of photoresist islands (240) is also removed or etchedlaterally or horizontally as the removal or etching process for theplanar substrate (210) progresses. The lateral removal rate for thephotoresist islands (240) may be much slower than the removal rate ofthe exposed portions of the planar substrate (210). The result of thisparticular feature of the removal process for the planar substrate (210)is the formation of an array of tapered nanopillars (250) having aconical form that is tapered thinner at the tip and wider at the basewhereby the base is on the planar substrate (210).

The applicants note that other removal processes for the planarsubstrate (210) can be used. In case the photoresist islands (240) arenot etched laterally or horizontally during the removal process for theplanar substrate (210), straight nanopillars rather than taperednanopillars would result. Arrays of straight nanopillars can also becovered by metal and be utilized as SERS structures.

By way of example and not of limitation, in an embodiment of the presentdisclosure, the tapered nanopillars (250) are 500 nm in height and madeof silicon dioxide material [Garda et al., 2010 Nanotechnology 21395701-395709]. As previously noted in the description of FIG. 2C in thepresent disclosure, the photoresist islands (240), and thus the taperednanopillars (250) can individually be approximately 150 nm in diameterat the base which is on the planar substrate (210) and 350 nm in pitch.Furthermore, the applicants note that for the tapered nanopillars (250)and other array structures suitable for SERS, higher periodicity isdesirable in creating a better enhancement factor with more hot spotsper area.

FIG. 2H shows the array of tapered nanopillars (250) after the remainingphotoresist islands (240) in FIG. 2G and the antireflective layer (230)in FIG. 2G are removed. The tapered nanopillars (250) may have sharptips on top. The resulting array of tapered nanopillars (250) from thisprocess is highly uniform, and can cover areas as large as a whole waferthat is many inches in diameter (e.g., 4, 6 or 12 inches). In anembodiment of the present disclosure, the resulting array covers a whole4-inch wafer [Garda et al., 2010 Nanotechnology 21 395701-395709].

FIG. 2I shows the array of tapered nanopillars (250) on the planarsubstrate (210) covered by a metal layer (260). The metal layer (260)covers both the array of tapered nanopillars (250) and the exposedportions of the planar substrate (210). The metal layer (260) can besilver, gold, aluminum, iridium platinum, palladium, copper or any othermetal. The covering by the metal layer (260) can be done byelectrochemical deposition, electron beam deposition (angled ororthogonal), physical sputtering, atomic layer deposition, or othermethods. In an embodiment of the present disclosure, the metal layer(260) is an 80 nm thick film of silver deposited by e-beam evaporation[Garda et al., 2010 Nanotechnology 21 395701-395709].

The array of tapered nanopillars (250) can be utilized as SERS substratefabricated with air filling in between the tapered nanopillars (250).Other dielectric materials, such as silicon dioxide can be used to coverand fill in between the tapered nanopillars (250) to tune the arraystructure suitable for SERS. The person skilled in the art will, again,understand that the number of steps shown in FIG. 2A-2I is onlyindicative and that the process can occur in more or fewer stepsaccording to the various embodiments.

FIG. 6 shows an example of a far field scanning confocal Ramanmicrospectroscopy system capable of far field SERS measurements toevaluate the performance of the array structure suitable for SERS. Thefar field scanning confocal Raman microspectroscopy system shown in FIG.6 comprises a spectrometer (610), a minor (620), a dichroic opticalelement (630), a collection lens (640), a laser (650), and a sampleplatform (660). In one embodiment of this disclosure, an averageenhancement factor of well into the 10⁷ range (see FIG. 7B) is measuredfor the array of tapered nanopillar (250) in FIG. 2H as SERS substrate,with a good repeatability over a 250 μm square region as shown in FIG.7A [Garda et al., 2010 Nanotechnology 21 395701-395709].

In the present disclosure, the applicants disclose two additionalmethods of characterizing the array structure suitable for SERS besidesthe far field scanning confocal Raman microspectroscopy. First, nearfield scanning optical microscopy (NSOM) is a microscopic technique fornanostructure investigation that breaks the far field resolution limitby directly detecting the near field emission or scattering. This isdone by placing the detector very close (where the distance is far lessthan the wavelength, typically a few nanometers) to the specimensurface. This allows for surface optical inspection with high spatial,spectral and temporal resolving power. By scanning the NSOM tip, the twodimensional Rayleigh scattering light tomograph can be obtained whichcan be directly compared to simulation results. The light enhancementfactor can also be acquired directly. Besides the near field Rayleighscattering measurement, the NSOM system can be coupled with a Ramanspectroscopy system to measure the SERS spectra in the near field.

Another potentially important method for characterizing the arraystructures suitable for SERS described in the present disclosure is theuse of local quantum-confined Stark effect. Although the NSOM can givethe scattering light profile at the SERS hot spot with typically tens ofnanometers in resolution, it is still important to measure the localelectric field with an even higher spatial resolution as the size of theSERS hot spot is usually smaller than 10 nanometers. A characterizationtechnique to obtain the accurate electric field information withpotentially nanometer spatial resolution is to place semiconductorquantum dots (QD) or artificial fluorophore molecules in the hot spot,and measure the QD bandgap change when applying optical excitation tothe SERS hot spot. The asymmetrical charge-carrier distribution functionin semiconductor materials favors holes in the direction of the appliedelectric field and electrons in the opposite direction, which leads to atilted band diagram. In this case the applied field leads to a reductionin the effective bandgap through the Stark effect.

By exploiting quantum-size effects in small scale structures, thereduction in the effective bandgap can be magnified in quantum dots, orquantum wells through the quantum-confined Stark effect (QCSE). Thebandgap condition of QD can be examined optically by photoluminescencemeasurement and/or fluorescence spectroscopy. The real local electricfield amplitude or intensity can be calculated from the measurementswith good confidence.

The examples set forth above are provided to give those of ordinaryskill in the art a complete disclosure and description of how to makeand use the embodiments of the present disclosure, and are not intendedto limit the scope of what the inventors regard as their disclosure.Modifications of the above-described modes for carrying out thedisclosure may be used by persons of skill in the art, and are intendedto be within the scope of the following claims. All patents andpublications mentioned in the specification may be indicative of thelevels of skill of those skilled in the art to which the disclosurepertains. All references cited in this disclosure are incorporated byreference to the same extent as if each reference had been incorporatedby reference in its entirety individually, even if the citation refersto only the first page of the reference.

It is to be understood that the disclosure is not limited to particularmethods or systems, which can, of course, vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting. As used in this specification and the appended claims, thesingular forms “a,” “an,” and “the” include plural referents unless thecontent clearly dictates otherwise. The term “plurality” includes two ormore referents unless the content clearly dictates otherwise. Unlessdefined otherwise, all technical and scientific terms used herein havethe same meaning as commonly understood by one of ordinary skill in theart to which the disclosure pertains.

A number of embodiments of the disclosure have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the presentdisclosure. Accordingly, other embodiments are within the scope of thefollowing claims.

1. A structure comprising: a planar substrate; an array of nanopillarson the planar substrate, the array of nanopillars substantiallyperpendicular to the planar substrate and having a substantiallycylindrical shape; a metal drop having a substantially spherical shapeon each nanopillar of the array of nanopillars; and a second metalcovering a surface of the planar substrate.
 2. The structure of claim 1,wherein the planar substrate and the array of nanopillars are selectedfrom the group consisting of: silicon dioxide, fused silica, sapphire,silicon, and compound semiconductors.
 3. The structure of claim 1,wherein the second metal is selected from the group consisting of:silver, gold, aluminum, iridium platinum, palladium, and copper.
 4. Thestructure of claim 1, wherein size, gap width and height of the array ofthe nanopillars and size, gap width and height of the metal drops on thearray of the nanopillars are configured to enable surface enhancement ofRaman scattering (SERS).
 5. The structure of claim 1, further comprisinga dielectric material covering and filling a region between the metaldrops and between the nanopillars.
 6. The structure of claim 1, whereina gap width between the metal drops on the array of nanopillars is lessthan approximately 50 nm.
 7. The structure of claim 1, wherein a gapwidth between the metal drops on the array of nanopillars is less thanapproximately 10 nm.
 8. The structure of claim 1, the structure being aSERS substrate.
 9. A structure comprising: a planar substrate; an arrayof nanorecesses in the planar substrate; and a first metal covering theplanar substrate and the array of nanorecesses.
 10. The structure ofclaim 9, wherein the planar substrate is selected from the groupconsisting of: silicon dioxide, fused silica, sapphire, silicon, andcompound semiconductors.
 11. The structure of claim 9, wherein the firstmetal is selected from the group consisting of: silver, gold, aluminum,iridium platinum, palladium, and copper.
 12. The structure of claim 9,further comprising a dielectric material covering and filling thenanorecesses.
 13. The structure of claim 9, wherein size, spacing, anddepth of the array of the nanorecesses and depth of the first metalcovering at least a portion of each nanorecess is configured to enablesurface enhancement of Raman scattering (SERS).
 14. The structure ofclaim 9, the structure being a SERS substrate.
 15. A structurecomprising: a planar substrate; an array of tapered nanopillars on theplanar substrate, wherein each tapered nanopillar of the array oftapered nanopillars is substantially perpendicular to the planarsubstrate and have a substantially conical shape, the conical shapebeing wider on a side connected with the planar substrate and tapered ona side opposite the planar substrate; and a metal layer covering asurface of the planar substrate and covering each tapered nanopillar ofthe array of tapered nanopillars.
 16. The structure of claim 15, whereinthe planar substrate and the array of tapered nanopillars are selectedfrom the group consisting of: silicon dioxide, fused silica, sapphire,silicon, and compound semiconductors.
 17. The structure of claim 15,wherein the metal layer is selected from the group consisting of:silver, gold, aluminum, iridium platinum, palladium, and copper.
 18. Thestructure of claim 15, further comprising a dielectric material coveringand filling in between the tapered nanopillars.
 19. The structure ofclaim 15, wherein size, pitch, and height of the array of taperednanopillar and thickness of the metal layer covering at least a portionof each tapered nanopillar is configured to enable surface enhancementof Raman scattering (SERS).
 20. The structure of claim 15, the structurebeing a SERS substrate.
 21. A structure comprising: a planar substrate;an array of straight nanopillars on the planar substrate, wherein eachstraight nanopillar of the array of straight nanopillars issubstantially perpendicular to the planar substrate and has asubstantially cylindrical shape; and a metal layer covering a surface ofthe planar substrate and covering each straight nanopillar of the arrayof straight nanopillars.
 22. The structure of claim 21, the structurebeing a SERS substrate.
 23. A method for fabricating a structure, themethod comprising: providing a planar substrate; coating the planarsubstrate with a polymer layer; annealing the polymer layer, wherein theannealing causes a microphase separation of the polymer layer into firstportions and second portions, the first portions being an array ofnanostructures surrounded by the second portions, the second portionsbeing a polymer matrix; removing the array of nanostructures, wherebythe removing forms an array of nanorecesses and the removing exposesportions of the planar substrate below the removed nanostructures; andcoating a first metal on the polymer matrix and the exposed portions ofthe planar substrate.
 24. The method of claim 23, wherein thenanostructures have a substantially spherical, cylindrical, or lamellarshape.
 25. The method of claim 23, wherein the polymer layer is blockcopolymer.
 26. The method of claim 23, wherein the polymer layer ispolystyrene-poly(methyl methacrylate).
 27. The method of claim 23,wherein the planar substrate is selected from the group consisting of:silicon dioxide, fused silica, sapphire, silicon, and compoundsemiconductors.
 28. The method of claim 23, wherein the first metal isselected from the group consisting of: silver, gold, aluminum, iridiumplatinum, palladium and copper.
 29. The method of claim 23, whereinsize, spacing, and height of the array of nanostructures and size,spacing, and depth of the first metal covering at least a portion ofeach nanorecess is configured to enable surface enhancement of Ramanscattering (SERS).
 30. The method of claim 23, wherein the structure isa SERS substrate.
 31. The method of claim 23, the method furthercomprising: removing the polymer matrix of the polymer layer and aportion of the first metal on the polymer matrix, wherein a remainingportion of the first metal forms an array of metal islands on the planarsubstrate; annealing the array of metal islands whereby the array ofmetal islands forms an array of metal drops on the planar substrate;removing portions of the planar substrate not covered by the array ofmetal drops whereby the removing forms a nanopillar under each metaldrop of the array of metal drops; coating a second metal on thesubstrate and the array of metal drops; and annealing the second metal,whereby the array of metal drops coated with the second metal undergoesshape change.
 32. The method of claim 31, wherein size, gap width, andheight of the array of the nanopillars and size, gap width and height ofthe metal drop of the array of metal drops is configured to enablesurface enhancement of Raman scattering (SERS).
 33. The method of claim31, wherein a gap width between the metal drops of the array of metaldrops is less than approximately 50 nm.
 34. The method of claim 31,wherein a gap width between the metal drops of the array of metal dropsis less than approximately 10 nm.
 35. The method of claim 31, whereinthe structures is a SERS substrate.
 36. A method for fabricating astructure, the method comprising: providing a planar substrate; coatinga photoresist layer on the planar substrate; exposing the photoresistlayer with laser light, the laser light forming a spatial interferencepattern on the photoresist layer; developing the photoresist layer toform an array of photoresist islands according to the spatialinterference pattern; removing portions of the planar substrate notcovered by the array of photoresist islands, wherein the removing formsan array of tapered nanopillars on the planar substrate; removing thearray of photoresist islands; and coating a metal layer on the array oftapered nanopillars.
 37. The method of claim 36, wherein size, pitch,and height of the array of tapered nanopillar and thickness of the metallayer is configured to enable surface enhancement of Raman scattering(SERS).
 38. The method of claim 36, wherein laser light is produced froma single laser light source, the single laser light source split intotwo laser beams, the two laser beams being recombined to form thespatial interference pattern.
 39. The method of claim 36, furthercomprising providing an antireflective layer on the planar substrate,wherein the coating the photoresist layer comprises coating anantireflective layer on the planar substrate and then coating thephotoresist layer on the antireflective layer; wherein the removingportions of the planar substrate comprises removing portions of theantireflective layer and portions of the planar substrate not covered bythe array of the photoresist islands; and wherein removing the array ofphotoresist islands comprises removing the array of photoresist islandsand the antireflective layer underlying the array of photoresistislands.
 40. The method of claim 36, wherein the structure is a SERSsubstrate.
 41. A method of measuring a light enhancement factor for anarray structure in surface enhanced Raman scattering (SERS), the methodcomprising: providing the structure according to claim 1; contacting aplurality of molecules with the structure, the plurality of moleculesbeing on the metal drop and on the second metal; and measuring a nearfield scattering intensity from the structure with a near field scanningoptical microscope, thus measuring the light enhancement factor for thestructure in surface enhanced Raman scattering (SERS).
 42. The method ofclaim 41, the structure being a SERS substrate.
 43. A method ofmeasuring a local electric field intensity with nanometer resolution foran array structure in surface enhanced Raman scattering (SERS), themethod comprising: providing the structure according to claim 1; placinga plurality of semiconductor quantum dots or artificial fluorophoremolecules in a plurality of hot spots in the structure; and measuring aphotoluminescence and/or a fluorescence spectroscopy and calculating theexpected local electric field intensity based on quantum-confined Starkeffect, thus measuring the local electric field intensity with nanometerresolution for the structure.
 44. The method of claim 43, the structurebeing a SERS substrate.